I will be recieving a xda ii mini(unlocked) same as the tmobile mda.I have the tmobile BIS plan on my blackberry.IF i was to insert my sim card into the xda phone will be able to browse the internet with the xda phone withought changing my data plan.

I had/have a BB 7100t - with the BB unlimited plan. I ended up buying an MDA because of various reasons. Not the least of which was the stupid goddamn policy Tmobile has of punishing existing users by not giving them the same deals as new users on the 8700.
The upshot of this is that I still have the BB unlimited plan and my MDA browses the web etc just fine using my old sim. Ultimately they point at exactly the same WAP but one has hotspots access. I couldnt care less about hotspots.

PS: I have to say I have been pleasantly surprised by the stability and functionality of the MDA, I do however miss the one handed use of the BB.